168 Dr Johannes Walther — The North American Deserts. 



and characteristic desert phenomena in North America and 

 Africa that present surprising resemblances. In the beginning 

 of October I rode from The Needles through the Mohave desert. 

 On both sides of the railway extended an almost horizontal 

 plain, gently rising toward the granitic and volcanic mountains. 

 So far as the eye could reach, I saw everything covered with 

 scattered desert shrubbery, sprinkling even the sloj^es of distant 

 mountains in the form of small green points. All mountains, 

 mostly volcanic rocks, dikes and ash-cones, rose island-like from 

 level desert land. The horizontal plain and the steep mountain 

 slopes were not linked together by a debris-covered foothill, but 

 plain and mountain slope intersected without any transition. 

 It is surprising to see steep mountainous islands rise from a sea 

 of debris, and yet this phenomenon is characteristic of all deserts 

 that I have seen in Africa, India and North America. Just as 

 the granite mountains of Sinai or of the Gharib rise island-like 

 from the debris plain in imposing dimensions, and as the plains 

 rise toward the base of the mountains so slowly as hardly to be 

 perceptible until the craggy granite colossus rears its head like 

 our own mountains of massive dolomite, so in the Sierra del 

 Diablo do the plateaus of the Carboniferous limestone rise steeply 

 from a boundless plain, of whose accumulated debris masses one 

 may form an idea on learning that near Torbert at the foot of the 

 Sierra Vanhorne a well was dug 1,050 feet deep in debris. The 

 phenomenon becomes especially striking, because it is noticed 

 that there are no debris deltas at the mouths of valleys 1,000 

 feet deep ; there, too, the horizontal plain is seen to abut directly 

 against the steep slopes of the mountains. 



If we conceive each landscape picture as the result of definite 

 processes of denudation, the relation of such a desert plain to its 

 rocky cliffs Avill at once indicate that denudation in the deserts 

 acts differently from what it does in Europe. But this horizon- 

 tality of the surfaces of denudation has a further claim on our 

 interest from another point of view. 



In the geologic exposures that exhibit to us sections through 

 parts of the earth's crust it is found very frequently that the rock 

 is parted by horizontal planes into layers lying above each other. 

 This structure is called stratification. Now that which in the 

 cross-section of a block of strata appears as a horizontal plane is 

 merely the expression of the fact that at a certain time in the 

 formation of that body of strata the freshly formed sedimentary 



